The advent of the laser (light amplification by the stimulated emission of radiation) in 1960 heralded a new era in scientific research because for the first time there became available a precisely controllable and very abundant source of photons, the quanta of radiation, which could be focussed into minute volumes (&lt;10.sup.-6 cm.sup.-3) of space to produce very high radiation energy densities in excess of 10.sup.12 ergs cm.sup.-3. However, many fundamental interactions involving photons, such as photon induced relativistic electrons, particle pairs and scattering events, require photon energy densities well in excess of 10.sup.12 ergs cm.sup.-3, in fact, the most interesting interactions will require photon energy densities in the range 10.sup.20 to 10.sup.30 ergs cm.sup.-3. It was soon realised during the early 1960s that major advances in laser technology would be required to achieve photon energy densities in excess of 10.sup.18 ergs cm.sup.-3. One of the required major advances in the field came in the form of the exponential amplifier (Hughes Allied Optics Vol. 6, page 1411, August 1967) which utilized divergent beams rather than the highly collimated beams of prior art systems.
To achieve the highest possible photon energy density in focussed laser beams, as much laser output energy as possible should be concentrated within the focal region of the beam using the shortest possible laser pulse duration and the best possible focussing optics.
The amplification of such short (&lt;10.sup.-9 seconds) duration pulses of laser energy within the laser amplifying medium is given by the relationship, EQU i = I.sub.o e.sup..alpha..chi. ( 1)
where I.sub.o is the intensity of the pulse entering the amplifier and I the intensity of the pulse after it has travelled a distance X cms in the laser amplifier medium of gain .alpha.cm.sup.-1. It will be seen from relation (1) that there is an exponential increase in the intensity of the laser pulse undergoing amplification.
If this situation was allowed to continue the photon energy density cm.sup.-2 of the pulse would soon exceed the destruction threshold of the amplifier medium. On the other hand, if the cross-sectional area of the amplifier was increased to accommodate the exponential increase of the pulse intensity, then it could be arranged that the pulse could be amplified at a constant flux density cm.sup.-2 through the amplifier. Such a laser amplifier is referred to as an "Exponential" amplifier and its overall gain is given by ##EQU1## It would be extremely difficult to construct an amplifier with exact exponential form so that its ideal characteristics can best be approximated to in practice by arranging a series of amplifying segments of increasing diameter in sequence. Such an approach is being followed in current high power laser systems although the rapid change to "Exponential" form has been somewhat suppressed by the fact that the larger diameter amplifier segments are limited to a circular beam diameter of about 20 cms due to parasitic self-oscillations within the segments which deplete their stored energy, accumulated over excitation pulse durations of 100 microseconds, prior to the arrival of the laser pulse to be amplified.
The power output of a laser amplifier can be approximated by the relationship, EQU P = (eA/t) watts (3)
where e is the safe loading of the laser medium, or its container, in joules cm.sup.-2, A the effective area of the output aperture in cm.sup.2 and t the duration of the laser output pulse. For continuous wave outputs t will be taken to be one second.
A major difficulty with high power laser systems is to minimise the destructive effects, e.g. self-focussing or beam inhomogeneities for large e/t ratios. For example, in present high power rod-disc neodymium doped glass laser systems e values lie in the range 0.1 to 1 joules cm.sup.-2 for t values ranging from 10.sup.-11 to 10.sup.-10 seconds, i.e. e/t values of between 10.sup.9 and 10.sup.10 watts cm.sup.-2 as a relatively safe operating range. Above these values, a real possibility exists of the beam self-focussing into minute filaments with excessive radiation loadings or becoming distorted to such an extent that the laser output cannot be focussed into a well defined focus volume.
If the possibility of improving the e/t ratio is neglected then the way to higher powers must involve the use of multiple laser beams or a very large single beam. The difficulty with multiple laser beams is the fact that it is extremely difficult to superimpose the focus region of each beam to increase the photon flux density, the unique exception being the superposition of two oppositely directed laser beams into a common focus region to produce an optical "centre-of-mass" region, an essential prerequisite to conserve energy-momentum in fundamental photon-photon and photon-particle (antiparticle) interactions. However, techniques have been firmly established to form such "centre-of-mass" regions using a single beam laser but one with a multiple pulse output. Here the first pulse is delayed and stored behind the common focus region of the focussing optics in such a manner that it returns into the common focus region as the second pulse out of the single beam laser enters the focus region for the first time. In this way, the two pulses can be made to overlap forming the required optical "centre-of-mass" interaction region. (J. L. Hughes, Proceedings of the VII International Quantum Electronic Conference, Journal of Quantum Electronics, Vol. QE-8, No. 6, page 536, 1972).
The situation has now been reached in the development of high power lasers where the required increases in the peak power output P relies to a greater extent on increasing the output area A because it is becoming increasingly difficult to improve the radiation loading e and to shorten the pulse duration t. However, great care has to be exercised in ensuring that the laser beam quality is maintained as A is increased otherwise the destructive effects of the e/t ratio will be enhanced at relatively low flux density cm.sup.-2 through the amplifier. Furthermore, as the aperture area A is increased, the parasitic self-oscillations in the segments or body of the amplifier segments must be suppressed otherwise it becomes pointless to increase A at all. This can be achieved by either selectively exciting the laser medium so that optimum excitation is attained in a period before the arrival of the laser pulse to be amplified, which of course must be less than the period required for the parasitic self-oscillations to build up to any significant level. Current technology does not allow the sequential excitation of laser amplifiers with large A values, i.e. greater than about 20 cm in beam diameter, because the sources for rapid excitation, i.e. photo-emitting diodes and particle beams cannot be economically justified on such a large scale at present. This mode of laser excitation is referred to as space sequential excitation to distinguish it from time sequential excitation. In time sequential excitation, the amplifier medium is brought up to near parasitic self-oscillation threshold with conventional excitation sources such as flashtubes or particle beam initiated discharges then rapidly excited in a period between 1 to 1000 nanoseconds (10.sup.-9 to 10.sup.-6 seconds) well above self-oscillation threshold with relatively very high efficiency (1% to 50%) so that the laser pulse could be amplified well before the parasitic self-oscillations set in. The large A valve output sections of high power laser systems can be successfully operated in this manner.
Another serious problem with the operation of high power laser systems is the fact that the spontaneous decay radiation, present to some extent in all excited laser media can also lead to parasitic self-oscillations of the laser system as a whole, i.e. along the axis of beam propogation rather than transverse to it as was the case discussed above for self-oscillations of large individual segments of such an amplifier. There are two basic approaches to the suppression of this axial self-oscillation of the laser system as a whole. Firstly, one can include very complex and expensive electro-optic, magneto-optic, switches into the path of the laser beam which are activated for a relatively short period to allow the passage of the laser pulse but suppress as much as possible the passage of the spontaneous radiation generated at the same wavelength as the laser pulse within the activated laser amplifier medium. However, it has to be stressed that the inclusion of any components in addition to the amplifier segments into the path of the laser beam will be detrimental to the laser system as a whole. A second, less costly in terms of component costs, and a passive version of the above switches is the self activity chemical dye switches. However, all these devices affect the quality of the laser beam and only the dye cell switch can be used with very large amplifier apertures A. Furthermore, the inclusion of large diameter dye cells in the path of the laser beam both reduces its intensity and also distorts its quality because it would be virtually impossible to guarantee high homogeneity of the liquid dye due to thermal gradient effects within the liquid dye medium.
A more subtle approach to the suppression of the spontaneous radiation along the length of the laser amplifier system is to utilize the fundamental difference between the spontaneous decay radiation and the coherent laser radiation, namely the fact that the latter is highly directional. Therefore, if the segments of the amplifier are well separated, there will be no loss of laser beam radiation but a drastic loss of the isotropically emitted spontaneous radiation. If we select a particular segment in the amplifier chain with, for example, A.about.10.sup.4 cm.sup.2, then the segments before and after it in the chain at a distance R cm will emit spontaneous decay radiation isotropically, i.e. over 2 radians so that a fraction A/2.pi.R.sup.2 falls on the particular segment under consideration from each of its neighbouring segments, i.e. a total of about A/.pi.R.sup.2 of the emitted spontaneous decay radiation. There will therefore be a loss of spontaneous decay radiation of about 8 .times. 10.sup.3 with A = 10.sup.4 cm.sup.2 and R = 5000 cm.